IR frequency calculations

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3:24:13 PM PDT - Fri, Aug 12th 2016
Hello All, I am having an equilibrated structure of a set of atoms from classical MD simulations. I want to calculate the IR frequencies for this particular structure obtained from MD as it is without performing any geometry optimization. Is there a way I can do that?? Thanks in advance, Aniruddha M Dive

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3:16:08 PM PDT - Tue, Aug 23rd 2016
Hi Aniruddha, Is your classical MD simulation in the condensed phase ? If so, you should be able to calculate the vibrational spectra with the information you have from the MD simulation. Basically, you calculate the autocorrelation function of the velocities. This will give you the vibrational density of states. This will include both IR and Raman modes. To calculate the IR specifically, you need to calculate the autocorrelation function of the molecular dipole. If you have a molecular system and if you want to calculate the IR at the DFT level of theory, then define the geometry in the geometry block and ask for task dft frequencies. See the following link for more information http://nwchemgit.github.io/index.php/Release66:Hessians_%26_Vibrational_Frequencies Hope this helps. Best, -Niri niri.govind@pnnl.gov